Modular, Off-Grid Electrochemical System for Disinfection of Irrigation Water and Disinfection/Ammonia Removal from Aquaculture Wastewater

Abstract

Although food supply in the United States is one of the safest in the world, there are about 48 million cases of foodborne illness and an estimated death toll of 3,000 each year due to food contamination by disease-causing bacteria (e.g., Shiga toxin-producing E. coli) or pathogens.[ 1 ] It is estimated that agricultural water is the most important route of contamination.[ 2 ] The U.S. Food and Drug Administration has established microbiological water quality criteria for irrigation and other farm uses (e.g., washing hands and produce).[ 3 ] These criteria are based on the presence of generic E. coli as indicators of the presence of fecal contamination. Similarly, disinfection and ammonia removal are two major challenges in water reuse and recycling in aquaculture and aquaponics systems, where water reuse is crucial in order to reduce water consumption and improve nutrient recycling.[ 4 ] Therefore, energy-efficient, modular, and scalable systems that can achieve disinfection and ammonia removal are of particular importance. Remote farms in particular require a system that can be integrated with renewable, off-grid energy supply.In this work, we developed electrochemical flow cells for both disinfection of irrigation water and disinfection/ammonia removal from aquaculture wastewater. Both synthetic waters and natural waters, which were collected from Hawaii, were tested and dosed with certain amounts of ammonia-nitrogen (NH3-N) and E. coli bacteria to obtain different contamination levels. For disinfection of irrigation water, graphite plates were used as both anode and cathode, and a significant disinfection effect was achieved when electric current was applied between the two electrodes. Hydrogen peroxide was detected in the solution and its concentration increased with electrolysis time. Therefore, the disinfection effect was attributable to hydrogen peroxide formed as an intermediate during oxygen reduction at the graphite cathode. For disinfection/ammonia removal from aquaculture wastewater, a PtRu/graphite electrode prepared by pulsed electrodeposition was used as the anode and a graphite plate was used as the cathode. Ammonia removal was slow in natural aquaculture water, which contains 0.66mM NaCl; however, the addition of low levels of NaCl (e.g., 5mM) can dramatically increase disinfection and ammonia removal rate. The concentration of free (HOCl, ClO-) and combined (NH2Cl, NHCl2, NCl3) chlorine was monitored during the experiment. It was found that before the complete removal of ammonia, free chlorine was not found in the solution, while during ammonia oxidation, a relatively stable concentration of combined chlorine was measured. However, after the complete removal of ammonia, combined chlorine was no longer detectable in the solution, while free chlorine appeared and its concentration increased with electrolysis time. These results indicate an active chlorine-mediated ammonia oxidation mechanism.[ 5 ] In this mechanism, chloride ions were oxidized at the anode to form free chlorine, which reacts with ammonia (or ammonium) to form combined chlorine. With more free chlorine being produced at the anode, the combined chlorine was further oxidized to nitrogen gas until the complete removal of both ammonia and combined chlorine. Another finding is that the solution’s pH decreased until the complete removal of ammonia, and then increased with electrolysis time. Therefore, pH could be used as an indicator for determining complete ammonia removal. More experimental findings, including monitoring the concentrations of nitrate, chlorate, and disinfection byproducts, and field demonstration tests in Hawaii, will be presented. An economical assessment of the flow cell system will also be discussed.